1. Field of the Invention
The present invention relates to a technique that drives a luminescent display panel that is equipped with, for example, an organic electroluminescent (EL) element as its luminescent element, and, more particularly, to a device for and a method of driving a luminescent display panel that can set the luminance of its EL element to a state that is suitable.
2. Description of the Related Art
As a display device that is low in power consumption, high in displayed quality, and can be thinned and that can be used instead of a liquid crystal display device, attention has been drawn toward an EL display device. On the background of this, there exists also the circumstance where the EL display device has progressively been streamlined, life-extended, and able to resist the practical use by using as the luminescent layer of the EL element used in the EL display device an organic compound from which good luminescent characteristics can be expected.
The organic EL element can electrically be expressed as an equivalent circuit such as that illustrated in FIG. 1. Namely, the organic EL element can be replaced with a construction of a parasitic capacitor component C and a diode component E that is connected in parallel to this capacitor component. The organic EL element therefore is thought to be a luminescent element with the property as a capacitance. When applied with a luminescent drive voltage, first, the organic EL element has entered into its electrode as the displacement current an electric charge corresponding to the electric capacitance of the element, and that electric charge is accumulated. Subsequently, when the resulting voltage has exceeded a prescribed voltage (the luminescent threshold voltage=Vth) specific for the element, an electric current starts to flow from the electrode (the anode side of the diode component E) into an organic layer constructing the luminescent layer. It can therefore be thought that luminescence occurs with an intensity that is proportionate to the electric current.
FIG. 2 illustrates the luminescence characteristic of the organic EL element. According to the luminescence characteristic, as illustrated in FIG. 2A, the organic EL element luminesces at a luminance (L) that is substantially proportionate to the drive current (I). As illustrated by a solid line in FIG. 2B, in case where the drive voltage (V) is equal to or higher than the luminescent threshold voltage (Vth), the electric current (I) rapidly flows, followed by luminescence. In other words, in case where the drive voltage is lower than the luminescent threshold voltage (Vth), almost no electric current flows into the EL element, followed by no luminescence. Accordingly, the luminance characteristic of the EL element has such a tendency as is that, as illustrated by a solid line in FIG. 2C, in the region of enabling luminescence where the relevant voltage is higher than the threshold voltage (Vth), the greater the value of the voltage (V) applied to the element is, the higher the luminance (L) becomes.
By the way, the above-described organic EL element has a characteristic that due to its long use the physical property of the element changes and the resistance value of the element itself becomes great. For this reason, as illustrated in FIG. 2B, in the organic EL element, the V-I characteristic thereof changes toward a direction indicated by the arrow (the characteristic indicated by a broken line) depending on the time period in which the element is put to practical use. Accordingly, the luminance characteristic also decreases. Also, the organic EL element has a problem, too, that the initial luminance thereof has a variation due to the variation in, for example, the deposition, as well, at the time of forming the relevant film. This is followed by the difficulty of expressing a luminance gradation that strictly corresponds to an input image signal.
For example, there has been proposed as one means for realizing a full-color display image by an organic EL element a parallel type RGB method wherein an organic material capable of causing the luminescence of red (R), green (G), and blue (B) color lights is separately formed and they are arrayed. In a full-color display device utilizing that RGB method, the totaled luminescing time period of a respective one of the R, G, and B elements is different, and, in addition, depending on the luminescent materials of the respective organic EL elements constituting the R, G, and B luminescent pixels, the speeds at which the respective values of luminance decrease are different. Therefore, the device has the problem that, with the passage of use time period, the color balance (white balance) after all collapses.
Further, it is also known that the luminance characteristic of the organic EL element generally changes with temperature in the way indicated by broken lines in FIG. 2C. Namely, while the EL element has such a tendency that, in the region of enabling luminescence where the relevant voltage is higher than the above-described luminescent threshold voltage, the greater the value of the voltage (V) applied thereto becomes, the higher the luminance (L) thereof becomes, the luminescent threshold voltage becomes lower as the temperature rises. Accordingly, the EL element is brought to a state of its luminescence being enabled with the voltage applied that is more decreased as the temperature increases. Therefore, the EL element has the dependency on temperature of luminance that, even if applied with the same luminescence-enabling voltage, when the temperature is high, the luminance is high and, when the temperature is low, the luminance is low.
Accordingly, in a case where realizing a full-color display image by the above-described parallel type RGB method, the device comes to have a problem that, due to the change in environmental temperature, as well, the color balance of R, G, and B similarly collapses.